1. Field of the Invention
The present invention relates to navigation systems. More, specifically, the present invention relates to torque motors for use in connection with gimbaled gyroscopes.
The present invention is described herein with reference to a particular embodiment for use in a particular application. As discussed more fully below, those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications and embodiments within the scope thereof.
2. Description of the Related Art
A torque motor is a motor that provides a controlled torque over an angular excursion to control the orientation of a gimbaled or free gyroscope in an inertial navigation system. As such devices are typically sensitive to disturbance torques, it is generally desirable that the torque motor have minimum spurious coupling torques. That is, the torque motor should operate with minimal magnetic reluctance torques (cogging) and with minimal magnetic hysteresis (friction).
Cogging is the undesirable tendency of a rotor to seek preferred positions and to cause torque pulsations (ripple torques during rotation). Cogging occurs when the edge of a rotor magnet is in the vicinity of a slot between the windings of the stator.
Friction is the undesireable tendency of the torque motor rotor to respond to residual magnetism in the stator. Thus, motion of the gimbal produces a torque between the rotor and stator in the command zero mode (i.e., when a zero torque command is present) and a dissipation of energy as heat when the rotor is commanded to apply a torque in one direction or another.
Two basic torque motor designs are the limited rotation design, made by Aeroflex, Vernitron and others, and the slot wound brush or electronically commutated design made by Inland, TRW Motors and others.
The limited rotation designs are characterized by a brushless DC motor wit a toroidally wound stator that provides ripple-free torque over a limited angular excursion without commutation. As no slots are utilized in this design, cogging is virtually eliminated. The rotor is usually well spaced from the stator allowing for minimum magnetic friction as well.
Unfortunately, limited rotation designs are typically power inefficient because part of the coil winding is often unused. In addition, such designs often suffer from transfer function nonlinearity (torque non-constancy as a function of angle) due to the variation in torque at the extremes of the range of motion.
Commutated or slotted designs are more similiar to conventional motors. As such, these designs typically exhibit good power performance because the windings may be bunched or concentrated in optimal locations. On the other hand, commutated or slotted designs exhibit the poor cogging and friction characteristics mentioned above. Also, such designs often suffer from transfer function nonlinearity due to ripple torques ie. the peak variations in torque resulting from the switching effect of commutation. Thus, there exists a general need in the art to provide a torque motor that exhibits good power efficiency, minimal cogging, minimal magnetic friction, and good transfer function linearity.